I will run some other tests when I get a chance. I've done #2, with no effect.
I don't think the Airport Expresses have anything to do with it - I've had some reports of other people being able to replicate my results without that.
Have you tried using specific channels (choosing channels with less interference from other access points in your area -- i use wifi-explorer) for 5ghz and 2.4ghz, instead of leaving them at automatic?
I have not in this case, but I've found in the past that the appropriate channels tend to shift with other usage in the area, and leaving it on automatic allows it to compensate for that, while hardcoding a channel can work for a while and then stop being the best.
